2. The new, close-up view from space
The Moon and Mercury demonstrate the power of impact
The most distinctive features on the Moon are the circular craters that closely pepper its face. Comparatively recent ones still exhibit the details of the impact that created them; older craters have been worn away by small particles that continuously bombard the Moon.
The lunar craters must have been created by solid, rocky objects, named meteoroids, that came from interplanetary space and hit the Moon. When the meteoroids strike the surface of a planet or satellite they are called meteorites. Although the projectile vaporizes on impact, the explosion excavates material and hurls it outward, creating a raised rim, radial ejecta and secondary craters. Meteorites of all sizes have hit the Moon, and its crust records the impact of more small meteorites than large ones.
The largest lunar craters are the impact basins. A typical one is the Imbrium Basin, with a diameter of 1.5 million meters. Its outline can be seen with the unaided eye, forming an ""eye socket"" of the face of the ""Man on the Moon"". Its outer rim is defined by prominent mountain ranges, such as the Apennine mountains. Such basins were created early in the Moon's history, and they were soon flooded and nearly filled with dark molten lava from the interior.
Mercury's surface also contains multi-ringed impact basins. The largest of these has been named Caloris, the Latin name for ""heat"", because it is located at a place on Mercury that faces the Sun when the planet is at the point in its orbit that is closest to the Sun. During the Mariner 10 encounters with Mercury, half of the Caloris Basin was in shadow and the other half in sunlight. An irregular annulus of mountains cuts across the sunlit image, defining the edge of a huge excavation that is 1.34 million meters in diameter.
The cataclysmic impact that created the Caloris Basin occurred an estimated 3.85 billion years ago when a meteorite roughly 150 thousand meters across hit Mercury, like a cosmic bomb with an energy of a trillion 1-megaton hydrogen bombs. The violent explosion reverberated through the young planet, sending strong seismic waves along the surface and through the deep interior. These waves converged to a focus on the side of Mercury opposite to the Caloris Basin, producing a peculiar terrain of cracks, faults, hills and valleys. The similarity of the surfaces of the Moon and Mercury, despite their differing masses and locations in the solar system, suggests that impacting objects were spread throughout the inner solar system during its early days. Mercury could have been bombarded at about the same time as the Moon, for scientists think that the entire solar system, with its Sun, planets and their satellites, formed 4.6 billion years ago.
Ubiquitous impact craters - from Mars and Venus to Callisto and Miranda
When Mariner 4 flew past Mars in 1965, snapping 22 close-up photographs, it revealed a wasteland riddled with the scars of an ancient rain of impacting meteorites. This heavily cratered part of the Martian surface, found in the red planet's southern hemisphere, has undergone little erosion over the aeons. Like the surfaces of Moon and Mercury, the oldest Martian terrain probably bears the scars of the intense cosmic bombardment during the first 500 million years of the solar system, as well as the marks of a continual bombardment since then.
Large impact craters on Venus are relatively scarce when compared with the closely-spaced, overlapping lunar craters. At one time Venus was probably as heavily cratered as the Moon, but the relatively small number and wide spacing of the craters now on Venus indicate that the surface we now see is much younger. When the Moon's cratering rate is scaled to Venus, the relative paucity of craters on its surface indicates an average age of about 750 million years, but the planet originated about 4.6 billion years ago. The relatively few craters we now see are due to meteoritic impact since the entire planet was resurfaced by rivers of outpouring lava about 750 million years ago.
Following impact, large objects left craters on Venus that at first sight resemble those on the Moon, with central peaks, flat floors, and distinct circular rims. But the dense atmosphere on Venus affected both the incoming projectile and its ejected debris, creating features that are unlike any other craters in the solar system. The bright apron of debris that surrounds large craters on Venus often has a lobate, petal-like appearance with an unexpected asymmetry. Material that was ejected from the crater became entrained in the hot, thick atmosphere, transforming it into a turbulent, fluid-like substance. The material flowed and spread out from the crater, creating patterns that resemble flowers or butterflies, rather than hurtling away from it to great distances.
Ancient impact scars are found on the rigid icy crusts of satellites in the cold outer parts of the planetary system. Jupiter's satellite Callisto, for example, has a rocky core surrounded by a deep layer of ice that is heavily scarred by impacts of meteorites. The icy moons of Saturn, such as Mimas and Tethys, are also heavily cratered. Further out we find Miranda, the satellite of Uranus, with the most bizarre surface of all. It has regions of distinctly different terrain. Some astronomers have argued that Miranda was once shattered into large fragments by a powerful collision, but that it managed to pull itself together again into a single body.
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Copyright 2010, Professor Kenneth R. Lang, Tufts University